In one approach to materials processing, deposition of material onto or removal of material from a substrate such as a semiconductor wafer is effected by subjecting the substrate to a processing medium, for example, a plasma, which is generated in situ from gaseous species and serves as the source of the reactive species required by the process. Typically, the reactors for such plasma-driven processes comprise opposing electrodes in a diode or triode configuration, energized so as to create the plasma in a controlled manner in an active region located between an electrode pair. The substrate to be treated is located in the active region near one of the electrodes.
This approach finds wide application, for example, in fabricating VLSI structures for integrated circuits. The desired dimensions of VLSI structures has continued to grow finer and finer. The shrinking scale of these structures has required that the gaseous species forming the plasma be present at lower and lower pressures. This demand for lower processing pressures is problematic for plasma-driven processes because of the critical role the pressure plays in determining the rate of deposition or removal of material. As the pressure is decreased to sufficiently low levels to satisfy the structure's dimensional requirements, the overall efficiency of the deposition or removal process becomes too low to fabricate devices quickly enough to be practical. The lower bound on pressure imposed by reaction-rate requirements has limited the use of plasma-driven processes to fabricating larger, coarser structures than would be otherwise desirable. In order to circumvent this limitation, techniques for improving the plasma density have been applied, so that a reasonable concentration of the desired ionic and neutral reactive species can be maintained at low pressures.
For example, plasma-phase reagent densities have been increased by providing external electrical or magnetic power to the reactor, additional to the usually applied rf bias power. This type of solution increases both the operating cost and complexity of the system. Another type of solution modifies the chamber design, specifically the electrode geometry, to allow an enhanced output without changing power requirements.
Such an electrode-geometry design is the hollow-anode glow discharge apparatus, described in U.S. Pat. No. 5,248,371, the entire disclosure of which is herein incorporated by reference. This patent describes the use of a grounded planar anode, opposing a parallel rf-powered electrode across the active region, perforated with holes to enhance the volume concentration of ions and other reactive species near the substrate. The electric field in the active region moves electrons away from the negatively-biased electrode toward the grounded anode. According to the '371 specification, some electrons enter holes in the anode; these may strike the walls of the holes and thereby cause the emission of secondary electrons, which in turn oscillate between the walls of the holes and collide with the walls or with species in the hole, creating cations and additional secondary electrons, respectively. The generation of cations and secondary electrons avalanches, thereby producing a "hollow-anode glow discharge" in each of the holes. This discharge affects the volume concentration of reactive species near the substrate and thereby enhances the reaction rate. The spatial extent of the hollow-anode glow discharge is by the annular plasma sheath around the periphery of the hole. Since the thickness of the sheath is inversely proportional to the system pressure, by increasing the hole size, a hollow-anode glow discharge can be maintained at lower pressures without changing the rf characteristics.
However, there appear to be limitations to this approach. For example, the hole size cannot be increased indefinitely without degrading the plasma density of the discharge. Likewise, the density can be enhanced by increasing the thickness of the anode over only a limited range, after which a thicker anode shows eroded density and even thicker anodes eventually extinguish the anode discharge. The inherent two-dimensionality of the perforated anode imposes a limit on the reaction rates it can contribute to the substrate.